This invention relates to a nonglaring mirror, i.e. variable reflectance mirror, which is functional as an electrochromic device
Known applications of liquid crystals include nonglaring mirrors That is, a liquid crystal cell having a transparent electrode film on the front side and a reflecting electrode film on the opposite side serves as a nonglaring mirror since reflectance of the mirror can be varied by application of an external electric field to change the state of the liquid crystal confined in a space between the two electrodes to thereby vary the light transmittance of the liquid crystal layer. However, practical use of liquid crystal mirrors is limited because of some disadvantages almost inherent to liquid crystals. In the case of a mirror using the light scattering effect of one type of liquid crystal, visibility of the mirror is significantly marred by bleeding of the reflected image when light transmittance of the liquid crystal is lowered. In the case of a mirror using the phase transition effect of another type of liquid crystal it is difficult to sufficiently lower the reflectance.
Recently much effort has been devoted to research and development of nonglaring mirrors using electrochromic effects. A nonglaring mirror is obtained by providing a highly reflecting surface to an electrochromic cell of the transmissive type. Early proposals include the use of a solution of an organic electrochromic material represented by viologen as the transmittance controlling material confined between a transparent electrode and a reflecting electrode. However, such devices have problems with operating temperatures and service life. Also it is well known to replace the aforementioned electrochromic solution by a combination of a film of a transient metal oxide, such as WO.sub.3 which colors blue in a reduced state, deposited on the transparent electrode and an electrolyte solution confined between the two electrodes. However, there is still a problem with the durability of the mirror because it is likely that a portion of the electrolyte solution undergoes an irreversible decomposition reaction at the surface of the electrode opposite the electrochromic oxide film.
In recently proposed electrochromic display devices which seem to be useful as nonglaring mirrors by the provision of a reflecting surface, it is common to coat the oppositely arranged two transparent electrode films with two different kinds of electrochromic materials, respectively. Also in these cases, the space between the two electrodes is filled with an electrolyte liquid. Typical combinations of two kinds of electrochromic materials are as follows.
According to Japanese patent application primary publication No. 55-64216 (1980), the first electrochromic material is a transition metal oxide which colors in a reduced state, such as WO.sub.3, and the second is a transition metal hydroxide which colors in an oxidized state, such as Ir(OH).sub.x. A disadvantage of a device or mirror using these electrochromatic materials is that coloring of the mirror is not deep enough because the transition metal hydroxide colors only palely so that the deepness of the mirror color is nearly equivalent to that of the transition metal oxide only.
According to Japanese patent application primary publication No. 59-155833 (1984), the first electrochromic material is a metal hexacyanometalate which is represented by M.sub.x [M'(CN).sub.6 ].sub.y (wherein M and M' are transition metals) and colors in an oxidized state, such as Prussian blue, and the second is a transition metal oxide which colors in a reduced state, such as WO.sub.3. There is a problem particular to any device using this combination of electrochromic materials. In producing the device it is inevitable that both the metal hexacyanometalate layer and the transition metal oxide layer are obtained in an oxidized state. That is, the former is in a colored state whereas the latter is in a bleached state. Therefore, it is necessary to perform an electrochemical treatment to reduce one of the two electrochromic layers before using the device. It is often occurs that the initial reduction treatment causes partial decomposition of moisture contained in its electrolyte liquid and oxidation of the electrochromic material in reduced state by the liberated oxygen. This is detrimental to the memory capability of the device. Besides, the decomposition of moisture is accompanied by some bubbling, which mars the appearance of the device. As a solution to this problem, Japanese patent application primary publication No. 59-159134 (1984) proposes to add an auxiliary electrode which comprises a reversibly oxidizable and reducible material. At tne initial reduction treatment the auxiliary electrode is used as the counter electrode. After that the auxiliary electrode serves no purpose. When the device is relatively small in size, the inutile space occupied by the auxiliary electrode becomes considerable compared with the effective coloring area. Besides, the provision of the auxiliary electrode raises the need of widening the distance between the two substrates coated with transparent electrode films and electrochromic layers.
According to Japanese patent application No. 58-188518 (1983), the first electrochromic material is a metal hexacyanometalate, such as Prussian blue, and the second is either a conjugated polymer which becomes lower in light transmittance in a reduced state, such as polypyrrole, or a metal oxyhydroxide which colors in a reduced state, such as NiO(OH). Since the conjugated polymers used in this proposal are colored whether in an oxidized state or in a reduced state, the device using any of such polymers does not become colorless and transparent when bleached and therefore is not suitable for many uses where transparency is required of the device in a bleached state. When a metal oxyhydroxide is used it is difficult to form a sufficiently thick layer of the second electrochromic material, and deepness of coloring of the device is insufficient because of paleness of the color of the metal oxyhydroxide.
For the above described reasons it is difficult to obtain fully practicable and sufficiently efficient nonglaring mirrors by using already developed or proposed electrochromic devices, while there is a strong demand for such nonglaring mirrors and particularly nonglaring rearview mirrors for automobiles.